The present invention relates to an apparatus for the production of polyurethane moldings which are endless in one dimension.
Endless polyurethane moldings are produced by continuous introduction of a reactive polyurethane mixture into continuous molds. The reactive mixture is prepared in mixheads, more particularly high-pressure mixheads, to which the components of the reactive mixture are delivered. Such mixheads typically have a mixing chamber in which the reactive components are mixed and an outlet through which the reactive mixture passes. A problem of the fully continuous operation is that deposits of reacted polyurethane mixture build up both in the mixing chamber and at the outlet of the mixhead so that production must be interrupted to clean the mixhead.
The production of polyurethanes is described in "Kunststoff-handbuch", edited by Becker/Braun, Vol. 7, "Polyurethane", 1993, pages 139-192; for mixheads, see pages 174 et seq.
Taking a mixhead with a discharge element for the reactive mixture out of operation and putting it back into operation is a complicated procedure. After the mixhead has been taken out of operation, it is important to ensure that no residues of reactive mixture are left in the mixing chamber and the outlet (or discharge element).
To this end, the supply of both reactive components to the mixhead is not stopped altogether. Instead only the supply of one component, generally the isocyanate component, is interrupted at first so that the other component, generally the polyol component, continues to flow through the mixhead and the discharge element for a certain time, so that residues of the reactive mixture are displaced from the mixhead and the discharge element (this is typically referred to as the "flush" mode).
Before the mixhead is put back into operation, it is important to ensure that the feed lines to the mixhead are free from air bubbles and that a correctly conditioned material, i.e. material with the necessary operating temperature and mixed in the correct ratios (for example polyol in the correct ratio), is present at the mixhead. To accomplish this, each component is circulated through the mixhead with the mixing chamber shut-off, the reactive components being passed through the mixhead and back to their respective storage tanks. Any air bubbles present in the feed lines are removed in this way and the material correctly conditioned by stirring and temperature control in the storage tank is passed continuously by the mixhead (this is typically referred to as the "conditioning" mode).
To avoid interruptions in operation for cleaning, repairing and replacing parts, it would be possible in principle to operate two mixheads alternately, each of the mixheads being equipped with separate, independent feed lines, pumps and control elements. The problem addressed by the present invention was to reduce the major complications this would involve and, in particular, to allow already existing equipment with only limited space for the fitting of additional units to be interchangeably equipped for the continuous operation of two mixheads alternately.
One solution to the above noted problem (i.e. to avoid interruptions in production during the cleaning and maintenance of the mixhead) has been proposed in published European Application 437,727. This reference describes providing two mixheads operated alternately, with both mixheads being charged by a common metering pump and with the components delivered first being passed for "conditioning" through the mixhead not in operation and then being delivered to the mixhead in operation through a permanently open connecting line between the two mixheads. Unfortunately, this proposal has the disadvantage that the direction of flow of the particular component in the permanently open connecting line must be reversed whenever operation is switched from one mixhead to the other which gives rise to pressure variations leading to metering variations in the mixing chamber. In addition, the mixhead in the "condition" mode cannot be dismantled because it is tied by the permanently open connecting line. Accordingly, full maintenance or replacement of mixhead elements during production, is not possible. A more serious disadvantage is that, during the "flush" mode with the polyol component, the formulation at the mixhead in the "mix" mode is no longer correct at all, so that satisfactory production is no longer possible.